External charger for a medical implantable device using field sensing coils to improve coupling

ABSTRACT

By incorporating magnetic field sensing coils in an external charger, it is possible to determine the position of an implantable device by sensing the reflected magnetic field from the implant. In one embodiment, two or more field sensing coils are arranged to sense the reflected magnetic field. By comparing the relative reflected magnetic field strengths of the sensing coils, the position of the implant relative to the external charger can be determined. Audio and/or visual feedback can then be communicated to the patient to allow the patient to improve the alignment of the charger.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/498,049, which is now U.S. Pat. No. 8,473,066, which is incorporatedherein by reference, and to which priority is claimed.

FIELD OF THE INVENTION

The present invention relates to techniques for providing improvedalignment between an external charger and an implantable device.

BACKGROUND

Implantable stimulation devices generate and deliver electrical stimulito body nerves and tissues for the therapy of various biologicaldisorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a Spinal Cord Stimulation (SCS) system, such asthat disclosed in U.S. patent application Ser. No. 11/177,503, which isnow U.S. Pat. No. 8,606,362 issued on Dec. 10, 2013.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. An SCS system typicallyincludes an Implantable Pulse Generator (IPG), electrodes, at least oneelectrode lead, and, optionally, at least one electrode lead extension.As shown in FIG. 1, the electrodes 106, which reside on a distal end ofthe electrode lead 102, are typically implanted along the dura 70 of thespinal cord 19, and the IPG 100 generates electrical pulses that aredelivered through the electrodes 106 to the nerve fibers within thespinal column 19. Electrodes 106 are arranged in a desired pattern andspacing to create an electrode array 110. Individual wires 112 withinone or more electrode leads 102 connect with each electrode 106 in thearray 110. The electrode lead(s) 102 exit the spinal column 19 and mayattach to one or more electrode lead extensions 120. The electrode leadextensions 120, in turn, are typically tunneled around the torso of thepatient to a subcutaneous pocket where the IPG 100 is implanted.Alternatively, the electrode lead 102 may directly connect with the IPG100.

As should be obvious, an IPG needs electrical power to function. Suchpower can be provided in several different ways, such as through the useof a rechargeable or non-rechargeable battery or through electromagnetic(EM) induction provided from an external charger, or from combinationsof these and other approaches, which are discussed in further detail inU.S. Pat. No. 6,553,263 (“the '263 patent”). Perhaps the favorite ofthese approaches is to use a rechargeable battery in the IPG, such as alithium-ion battery or a lithium-ion polymer battery. Such arechargeable battery can generally supply sufficient power to run an IPGfor a sufficient period (e.g., a day or more) between recharging.Recharging can occur through the use of EM induction, in which EM fieldsare sent by an external charger to the IPG. Thus, when the battery needsrecharging, the patient in which the IPG is implanted can activate theexternal charger to transcutaneously (i.e., through the patient's flesh)charge the battery (e.g., at night when the patient is sleeping orduring other convenient periods).

The basics of such a system are shown in FIG. 2. As shown, the systemcomprises, in relevant part, the external charger 208 and IPG 100. Aprimary coil 130 in the charger 208 produces an EM field 290 capable oftranscutaneous transmission through a patient's flesh 278. The externalcharger 208 may be powered by any known means, such as via a battery orby plugging into a wall outlet, for example. The EM field 290 is met atthe IPG 100 by another coil 270, and accordingly, an AC voltage isinduced in that coil 270. This AC voltage in turn is rectified to a DCvoltage at a rectifier 682, which may comprise a standard bridgecircuit. (There may additionally be data telemetry associated with theEM field 290, but this detail is ignored as impertinent to the presentdisclosure). The rectified DC voltage is, in turn, sent to a chargecontroller and protection circuit 684, which operates generally toregulate the DC voltage and to produce either a constant voltage orconstant current output as necessary for recharging the battery 180.

FIG. 3 shows further details of external charger 208 with the topportion of the housing removed. Further details concerning externalchargers can be found in U.S. patent application Ser. No. 11/460,955,filed Jul. 28, 2006. As shown in FIG. 3, electrical current 114 flowingin a counterclockwise direction through the primary coil 130 induces amagnetic field 290 having a prominent portion in a directionperpendicular to the plane in which the primary coil 130 lies. Primarycoil 130 is typically formed of many turns of copper Litz wire, but theindividual turns are not shown in FIG. 3 for clarity. Thus, when a faceof the case of the external charger 208 is oriented in close proximityto an implanted device, such that the primary coil 130 is parallel to acorresponding coil within the IPG 100, the magnetic field generated bythe primary coil 130 induces an electrical current within acorresponding coil to charge a battery within, or otherwise providepower, to the IPG 100.

This system is akin to a transformer where the primary coil is in theexternal charger 208 and secondary coil in the IPG 100. The efficiencyof this coupling is largely dependent upon the alignment between the twocoils, which efficiency can be expressed as a coupling factor, k.Achieving a good coupling factor is essential for optimizing efficiencyof the inductive link. Not only does good coupling increase the powertransferred to the implant, it minimizes heating in the implant, andalso reduces the power requirements of the external charger, whichreduces heating of the charger and allows a smaller form factor. Propercoupling is also essential if there is to be any data telemetry betweenthe external charger 208 and the implant.

Operation of the external charger 208 in the prior art typicallyinvolves the use of audio feedback to the user. Thus, when chargingbegins, the external charger 208 produces induced field 290 and beginssearching for the IPG 100, as will be explained in more detail herein.An audio transducer in the external charger 208 would provide anintermittent audible sound (e.g., beeping) when coupling was poorbetween the charger 208 and the IPG 100, which beeping would alert theuser to move the external charger relative to the IPG. Once thepositioning and coupling were improved, the charger 208 would stopbeeping, and the location of the charger 208 would be held in place overthe IPG 100 by using double-side adhesive pads or a belt. If the charger208 again became poorly positioned relative to the IPG 100, the audiotransducer would again start beeping, so that the position of thecharger 208 relative to the IPG 100 could again be readjusted. Aback-telemetry link from the IPG 100 would communicate to the charger208 when the IPG battery was fully charged, which condition can again beaudibly signaled to the patient.

As noted earlier, proper alignment between an external charger and animplant is essential for proper system function, energy transfer, andsafety to the patient. However, this has heretofore been difficult toachieve. In particular, it has been noticed by the inventors that it isdifficult for prior art external chargers to differentiate between adeeply-implanted device that is well aligned with respect to thecharger, and a shallowly-implanted device that is poorly aligned withrespect to the charger. Either scenario appears the same to the externalcharger 208. As a result, the patient will only know that the couplingis poor, but will not know how to remedy this situation apart fromtrial-and-error re-positioning of the charger.

Given these shortcomings, the art of implantable devices would benefitfrom techniques for achieving improved coupling between an externalcharger and an implantable device that provide: the ability toaccurately indicate the relative position of the charger to the implant;increased charging efficiency; faster charging rates; increased patientsafety and comfort; lower power requirements; and a smaller form factor.This disclosure presents such a solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an implantable pulse generator (IPG), an external charger,and the manner in which an electrode array is coupled to the IPG, inaccordance with the prior art.

FIG. 2 illustrates a prior art system comprising an external charger forcharging an implantable pulse generator, including the charge controllerand battery protection aspects of the IPG.

FIG. 3 shows a perspective view of a prior art external charger for animplantable medical device.

FIGS. 4A-4C illustrate typical configurations, wherein the primary coilof a prior art external charging device is located at or near the outersurface of the patient's skin and the secondary coil of an implantablemedical device is located near to or far from the inner surface of thepatient's skin.

FIG. 5A shows a perspective view of one possible embodiment of animproved external charger for an implantable medical device.

FIG. 5B shows a typical configuration, wherein the primary coil of animproved external charging device is located at or near the outersurface of the patient's skin and the secondary coil of an implantablemedical device is located near the inner surface of the patient's skin.

FIG. 5C shows two sensing coils whose outputs are sent to aninstrumentation amplifier.

FIG. 5D shows a block diagram of circuitry for a system comprising animproved external charger for an implantable medical device.

FIG. 5E shows two sensing coils connected in series and end-to-end.

FIG. 5F shows a block diagram of circuitry for a system comprising animproved external charger for an implantable medical device.

FIGS. 5G-5H illustrate typical configurations, wherein the primary coilof an improved external charging device is located at or near the outersurface of the patient's skin and the secondary coil of an implantablemedical device is located near to or far from the inner surface of thepatient's skin.

FIG. 6A shows a perspective view of another embodiment of an improvedexternal charger for an implantable medical device.

FIG. 6B shows a circuit diagram for a system comprising an improvedexternal charger for an implantable medical device.

FIG. 6C shows a top down view of a system comprising an improvedexternal charger for an implantable medical device.

FIG. 7 shows a perspective view of one possible embodiment of animproved external charger for an implantable medical device.

FIG. 8 shows a system comprising an improved external charger forcharging an implantable pulse generator, including the alignment sensingand position indication circuitry of the external charger.

FIG. 9 is a flowchart detailing one embodiment of a technique forassuring the proper alignment of an external charger to an IPG.

FIG. 10 shows one embodiment of an improved external charger forcharging an implantable pulse generator.

DETAILED DESCRIPTION

The description that follows relates to use of the invention within aspinal cord stimulation (SCS) system. However, it is to be understoodthat the invention is not so limited. Rather, the invention may be usedwith any type of implantable medical device that could benefit fromimproved alignment between an external charger and the implantabledevice. For example, the present invention may be used as part of asystem employing an external charger configured to charge a pacemaker,an implantable pump, a defibrillator, a cochlear stimulator, a retinalstimulator, a stimulator configured to produce coordinated limbmovement, a cortical or deep brain stimulator, or in any otherstimulator configured to treat urinary incontinence, sleep apnea,shoulder sublaxation, etc. Moreover, the technique can be used innon-medical and/or non-implantable devices or systems as well, i.e., inany device or system in which proper coupling between a primary andsecond device is necessary or desirable.

As noted earlier, achieving proper coupling between an external chargerand an implant can be difficult, as it is hard for the external chargerto differentiate between a deep implant that is well aligned to theexternal charger and a shallow implant that is misaligned with theexternal charger. Both scenarios appear the same to the externalcharger. The present invention provides an improved external chargerhaving improved means for determining the position of the implanteddevice relative to the charger by sensing the magnetic field reflectedfrom the implanted device.

In one embodiment, the external charger 208 contains magnetic fieldsensing coils to help discriminate between deep implants and misalignedimplants. Through use of these magnetic field sensing coils, it ispossible to determine the position of an implantable device by sensingthe reflected magnetic field from the implant. In one embodiment, threeor more field sensing coils are arranged within the charge coil in aplane or planes parallel to the charge coil. In another embodiment, twoor more field sensing coils are arranged within the charge coil in oneor more planes perpendicular to the charge coil. By comparing therelative reflected magnetic field strengths of the sensing coils, theposition of the implant can be determined. Audio and/or visual feedbackcan then be communicated to the patient to allow the patient to improvethe alignment of the charger.

FIG. 4A shows a primary coil 130 configured for transcutaneouslycharging the IPG 100 via inductive coupling in accordance with the priorart. As mentioned earlier, the charger 208 comprises a primary coil 130,through which an AC current 114 is passed via an AC current source 170.This current 114 produces induced magnetic field 290, which isillustrated as a plurality of flux lines 160. Flux lines 160 areessentially perpendicular to the surface of the skin 278 where they passthrough its surface. In addition, the magnetic flux lines 160 near thecenter of the primary coil 130 are substantially parallel to the centralaxis 275 of the coil. A corresponding coil 270 within the IPG 100transforms this magnetic energy into an electrical current, which isrectified and used by circuitry to charge a battery 180 within the IPG100 as explained previously. The distance between the charger 208 andthe IPG 100 is typically on the order of about 1-5 centimeters.

The primary and secondary coils 130 and 270 are substantially in theshape of a circular loop, and are typically formed of several turns ofwire, as one skilled in the art will appreciate. However, it will berecognized that the substantially circular shape of the coils 130 and270 are merely illustrative. The turns of the primary coil 130 define acenter opening or aperture having a central axis 275. It will berecognized that the surface of the skin 278 is not always flat. Hence,the central axis 275 of the primary coil 130 is sometimes onlyapproximately or substantially perpendicular to the surface of the skin278.

The induced magnetic field 290 produces eddy currents in the IPG'stypically metallic case 101 or in other conductive structures within theIPG 100. Such eddy currents operate to produce a reflected magneticfield 295, which operates to change the mutual inductance of the primarycoil 130, effectively “detuning” the coil. Such detuning changes Vcoil,the voltage used to produce the current in the primary coil 130.Accordingly, from monitoring Vcoil, the relative coupling between theexternal charger 208 and the IPG 100 can be inferred. Vcoil decreases asthe coupling increases, which generally occurs when the external charger208 and the IPG 100 are closer to one another.

However, this means of monitoring coupling between the external charger208 and the IPG 100 cannot discern between distance and misalignment,which conditions are illustrated in FIGS. 4B and 4C. FIG. 4B shows anIPG 100 implanted relatively deeply within a patient, but otherwise wellaligned from an axial perspective, i.e., coil axes 275 and 276 (see FIG.4A) are not offset from each other. FIG. 4C, by contrast, shows an IPG100 implanted relatively shallowly with a patient, but with pooralignment, i.e., coil axes 275 and 276 (see FIG. 4A) are offset to alarge degree. In either of these cases, the coupling between theexternal charger and the IPG 100 will be relatively poor, with theresult that Vcoil will not be greatly affected by the IPG 100. However,because Vcoil might be the same in magnitude for both conditions, Vcoilcannot be used to discern between depth (FIG. 4B) and misalignment (FIG.4C). As a result, Vcoil cannot be used by the external charger 208—andultimately the patient—to qualify the reason for poor coupling, or howto fix the poor coupling by appropriate repositioning of the externalcharger 208.

FIG. 5A shows one embodiment of an improved external charger 210 withthe ability to determine the relative position of an implanted device,and thus maximize coupling by indicating to the user how to improvecharger/device alignment. In this embodiment, four field sensing coils230 are arranged into two pairs, 230 x and 230 y, of two connectedsensing coils each. Each of the pairs of sensing coils 230 x and 230 yare positioned within primary coil 130 and such that the plurality ofsensing coils are wound around axes that are parallel to the centralaxis 275 (FIG. 5B).

The field sensing coils 230 are designed to detect the amount of thereflected magnetic field 295 passing through them. Each pair 230 x and230 y straddles the central axis 275 (FIG. 5B) of the primary coil 130,such that the coils in each pair are equidistant from the central axis275 and opposite each other. As shown, the pairs 230 x and 230 y arepositioned orthogonally with respect to each other. FIG. 5B shows aprimary coil 130 configured for transcutaneously charging the IPG 100via inductive coupling with sensing coils 230 arranged in accordancewith the embodiment shown in FIG. 5A. As is explained in greater detailbelow, by comparing electrical measurements, such as the reflectedmagnetic field strengths induced in each sensing coil 230 of each pairof sensing coils, 230 x and 230 y, the position of the implant in boththe x- and y-directions can be determined by the external charger 210'sposition indication circuitry 279 (FIGS. 5C-5F). Audio and/or visualfeedback of the implant position can then be communicated to the patientto improve alignment of the charger.

In the embodiment of FIG. 5C, alignment of the primary coil 130 relativeto the IPG 100 is determined by alignment sensing circuitry 281. Theoutput of alignment sensing circuitry 281, comprising x- and y-errorvoltages 272 x and 272 y, is provided to a position indication circuitry279, which indicates to the user the misalignment of the externalcharger 210 relative to the implantable medical device. Such means ofindicating misalignment to the user will be discussed further below.

FIG. 5C shows one potential arrangement of a pair of sensing coils 230 xfor the improved external charger 210 that is depicted in FIGS. 5A and5B. In this embodiment, only sensing coils 230 x 1 and 230 x 2—which areused to determine the IPG 100's misalignment with the external charger210 in the x-direction—are shown for the sake of simplicity. A completeexternal charger 210 utilizing this embodiment will also have acorresponding pair of sensing coils 230 y 1 and 230 y 2 to measuremisalignment in the y-direction, as is seen in FIG. 5D and explained infurther detail below. In the embodiment of FIG. 5C, the sensing coils230 x and 230 y are not connected to each other, i.e., each sensing coil230 x has one terminal connected to ground and the other terminalconnected to a detector 274 that outputs a signal indicative of thevoltage measured at each sensing coil 230 x. Each detector 274 may beimplemented as a half-wave rectifier using a single diode, for example.

The output signal from each detector 274 in a sensing coil pair, e.g.,VcoilX1 and VcoilX2, is then sent to an instrumentation amplifier 273 xwhich, as is known to one of skill in the art, amplifies the differencebetween the two signals fed into it. The output 272 x of theinstrumentation amplifier 273 x is an error indication signal, alsoknown as an “error voltage.” The error voltages indicate alignment ofthe external charger 210 and the implantable medical device 100 withrespect to a particular direction. In the case of x-error voltages andy-error voltages, the directions are perpendicular to each other. In theembodiment of FIGS. 5C and 5D, the voltages across each sensing coilwithin each sensing coil pair are compared to each other to produce anerror voltage with respect to a particular direction. In other words,alignment sensing circuitry 281 derives a first indicator and a secondindicator, wherein the first and second indicators indicate misalignmentwith respect to first and second directions. The outputs 272 x and 272 yare then converted from analog signals into digital signals and thensent to the improved external charger 210's position indicationcircuitry 279 so that the location of the implantable medical device 100can be determined and appropriate instruction can be delivered to theuser as to how to improve the external charger 210's alignment with theimplantable medical device 100.

FIG. 5D shows a circuit diagram depicting the alignment sensingcircuitry 281 of the improved external charger 210 depicted in FIG. 5C.As mentioned above in reference to FIG. 5C, each sensing coil 230 in theexternal charger 210 is connected to a detector 274. The output signalfrom each detector 274 in a “sensing coil pair,” e.g., the detectorsmeasuring the voltage at sensing coils 230 x 1 and 230 x 2 (which outputsignals are labeled in FIG. 5D as VoilX1 and VcoilX2, respectively), isthen sent to an instrumentation amplifier 273 x which, as explainedabove, amplifies the difference between the two signals fed into it.

If IPG 100 is closer in the x-direction to sensing coil 230 x 1 than itis to sensing coil 230 x 2, the voltage detected at sensing coil 230 x 1will be lower, say 50V, than the voltage detected at sensing coil 230 x2, say 52V. This difference of positive two volts (VcoilX2−VcoilX1) willcause instrumentation amplifier 273 x to output a positive voltagesignal. If instead, the IPG 100 is closer in the x-direction to sensingcoil 230 x 2, the voltage at sensing coil 230 x 1 will be higher, say52V, than the voltage detected at sensing coil 230 x 2, say 50V. In thiscase, the difference of negative two volts will cause instrumentationamplifier 273 x to output a negative voltage signal. In other words, themagnitude of the signal output by instrumentation amplifier 273 x isdirectly proportional to the difference in magnitude between VcoilX1 andVcoilX2. The magnitude of the difference also indicates relativecloseness of the primary coil 130 and the IPG 100. For example, if thevoltages measured at 230 x 1 and 230 x 2 were 45V and 57V, respectively,instead of 50V and 52V as in the example above, the difference betweenthe signals would be 12V, and the magnitude of the signal output byinstrumentation amplifier 273 x would be greater than in the 52V/50Vexample. The greater output by instrumentation amplifier 273 x in the45V/57V example would indicate to position indication circuitry 279 thatIPG 100 was located even further towards sensing coil 230 x 1 in the45V/57V scenario than it was in the 50V/52V scenario. Thus, thisembodiment is able to provide detailed information about the IPG 100'srelative location in the x-direction. As will be understood, the sameprocessing is simultaneously being carried out by sensing coils 230 y 1and 230 y 2 to determine the IPG 100's relative location in they-direction, thus allowing external charger 210 to give a completepicture of IPG 100's location.

However, because the sensing coils 230 in this embodiment are arrangedin the same plane as the primary coil 130, the measured magnetic fieldstrength will have a large bias due to coupling from the primary coil130. In the example given above with respect to FIG. 5D, the differencebetween 52V and 50V is not very large (i.e., 2V) when compared to theabsolute voltages being measured on the sensing coils. Thus, it can bedifficult to rapidly calculate the difference between the voltages ofthe two sensing coils with a high degree of resolution.

Another embodiment, as shown in FIGS. 5E and 5F, presents a solution tothis issue. Specifically, the sensing coils 230 x 1 and 230 x 2 in thisembodiment are connected to each other in series and “end-to-end.” Theterm ‘end-to-end,’ as used herein, simply means that the end of onesensing coil, i.e., the ‘B’ Terminal as shown in FIG. 5E, is connectedto the end, i.e., ‘B’ Terminal, of another sensing coil. Alternatively,the beginning of one sensing coil, i.e., the ‘A’ Terminal as shown inFIG. 5E, could be connected to the beginning, i.e., ‘A’ Terminal, ofanother sensing coil. The terms “beginning” and “end”—and thedesignations ‘A’ Terminal and ‘B’ Terminal—are relative to the sensingcoil, and are defined by the direction of the induced magnetic field,which should be the same for each sensing coil in a sensing coil pair.Connecting the sensing coils end-to-end in that manner cancels out thecommon mode AC voltage between sensing coil 230 x 1 and sensing coil 230x 2. In other words, the voltage measured on the ‘A’ terminal of sensingcoil 230 x 1 will be the difference of the voltages across sensing coils230 x 1 and 230 x 2. The manner in which the coils are connectedessentially performs the work of the instrumentation amplifier 273,which was described above in reference to FIGS. 5C and 5D, thusobviating the need for instrumentation amplifiers in this embodiment.

With respect to the 52V/50V example given above in relation to theembodiment of FIGS. 5C and 5D, the embodiment of FIGS. 5E and 5F wouldsimply output a 2V signal to synchronous detector 284. This would veryclearly be a non-zero signal, indicating misalignment of the externalcharger 210 and the IPG 100. There would be no need to compare twodifferent, relatively large voltage measurements and amplify theresulting difference with enough resolution to get meaningfulinformation, as was done in the embodiment of FIGS. 5C and 5D. The onlymeasurement that needs to be taken in the embodiment of FIGS. 5E and 5Fis the magnitude of the output voltage for each sensing coil pair. Aswould be expected, this embodiment would result in a net voltage of zeroat synchronous detector 284 when IPG 100 is centered symmetrically withrespect to sensing coils 230 x 1 and 230 x 2. As would be understood byone of skill in the art, synchronous detector 284 will also need to beconnected to a timing reference signal, which is not shown forsimplicity.

The outputs 272 x and 272 y of each synchronous detector 284 x and 284y, are error indication signals, also known as an “error voltages.” Theerror voltages indicate alignment of the external charger 210 and theimplantable medical device 100 with respect to a particular direction.In the case of x-error voltages and y-error voltages, the directions areperpendicular to each other. In this embodiment, the voltages acrosseach sensing coil pair are measured to produce an error voltage withrespect to a particular direction. The error voltages are then convertedfrom an analog signal into a digital signal and sent to the improvedexternal charger 210's position indication circuitry 279 so that thelocation of the implant 100 can be determined and appropriateinstructions can be delivered to the user as to how to improve thecharger 210's alignment with IPG 100.

FIG. 5G shows a scenario where an implantable medical device 100 isdeeply implanted in the patient's body but well aligned with externalcharger 210. In this scenario, each of the field sensing coils 230 wouldhave a similar Vcoil because each coil would pick up an equivalentreflected magnetic flux, i.e., VcoilX1, VcoilX2, VcoilY1, and VcoilY2would all be equal. Thus, the differences between the measured voltagesof the two sensing coils in each of the sensing coil pairs would beclose to zero, and the position indication circuitry 279 would determinethat the external charger 210 was properly, i.e., symmetrically, alignedwith the IPG 100.

FIG. 5H shows a scenario where an implantable medical device 100 isshallowly implanted in the patient's body but poorly aligned withexternal charger 210; specifically, it is skewed in the y-direction. Asdiscussed above, a prior art external charger would not be able todistinguish between the scenario presented in FIG. 5G and the scenariopresented in FIG. 5H. However, with the improved external charger 210,these two scenarios are distinguishable. In the scenario shown in FIG.5H, field sensing coil 230 y 2 measures a lower Vcoil (VcoilY2) thansensing coil 230 y 1 (VcoilY1) because sensing coil 230 y 2 picks up adisproportionately larger amount of reflected magnetic flux. As wasdiscussed above, the alignment sensing circuitry 281 compares theVcoilY1 and VcoilY2 values. In this scenario, it would result in adetermination that VcoilY2 is smaller than VcoilY1 and alignment sensingcircuitry 281 would output a negative “Y-error voltage” value 272 y.This signal would then be converted into a digital signal, sent toposition indication circuitry 279, and interpreted to mean that the IPG100 was actually closer to sensing coil 230 y 2 than it was to sensingcoil 230 y 1, i.e., that the charger 210 was too far to the left asillustrated. The external charger 210 would then indicate to the userhow to correct the alignment problem, i.e., by instructing the user tomove the charger 210 to the right, to maximize the electrical couplingof external charger 210 and implantable medical device 100. The sameprocessing is simultaneously carried out by sensing coils 230 x 1 and230 x 2 to report information about the IPG 100's location in thex-direction.

Sometimes, the reflected magnetic field 295 from the implanted device100 is not very strong, especially when the implanted device 100 isimplanted deeply within the patient. Thus, it can be difficult to detectvery small differences in the voltages across each of the sensing coils230. The resolution of the measurements may have to be high for thecircuitry to be able to notice the small differences in voltage.Additionally, due to the constantly changing nature of the electricfield caused by the user's breathing and heart beat, the signal from allsensing coils 230 should be measured nearly simultaneously for a propercomparison.

FIG. 6A shows an alternative embodiment of an improved external charger210 with the ability to determine the position of an implanted device.In this embodiment, two or more cylindrical field sensing coils 240 a-bare arranged within the primary coil 130 and wound around axesperpendicular to the central axis 275 of primary coil 130. Since theaxes of the sensing coils 240 a-b are perpendicular to the axis of theprimary coil 130, coupling between the primary coil 130 and the sensingcoils 240 a-b is mitigated. The sensing coils 240 a-b are therefore morelikely to be affected by the magnetic fields reflected from the implant100. When a sensing coil 240 is placed with its axis perpendicular tothe direction of the magnetic field, i.e., in perfect alignment with theimplanted device 100, the voltage across sensing coils 240 a and 240 bequals zero. With misalignment, however, the reflected magnetic field295 contains components that are tangential to the axis of the sensingcoils 240 a-b, resulting in a non-zero voltage. To measure thistangential component and, by this measurement, to estimate amisalignment, it is preferred to place sensing coils 240 a-b in thecenter of primary coil 130, with their axes lying in the primary coil130 plane and oriented to measure misalignment in a particular axialdirection. Thus, one sensing coil 240 b is needed to detect themisalignment of the implant 100 in the x-direction, and another sensingcoil 240 a is needed to detect the misalignment of the implant 100 inthe y-direction. For mechanical reasons discussed below with referenceto FIG. 6C, it is helpful to divide the sensing coil 240 a and 240 b foreach axial measurement into two separate parts and then connect theseparate parts to each other using wires 244 a and 244 b. This way, onlya single coil voltage needs be measured for each axial direction. Bycomparing these voltages, the position of the implant 100 can bedetermined. The coils may be constructed as, for example, air-core coilsor ferrite-core coils. If the field sensing coils 240 a-b are centereddirectly over implant 100, zero volts will be registered by thealignment sensing circuitry 281 (FIG. 6B) of the external charger 210.The role of insert piece 241 and cylinder holders 242 and 243 will bediscussed in further detail below with regard to FIG. 6C.

FIG. 6B shows a circuit diagram depicting the alignment sensingcircuitry 281 of the improved external charger 210 depicted in FIG. 6A.Primary coil 130 is powered by amplifier 252, which is excited byoscillator 251. A pair of mixers 254 a and 254 b, which are referencedto the excitation voltage, Ve, are used to detect the direction of themagnetic flux going through field sensing coils 240 a and 240 b,respectively. Because the voltage across field sensing coils 240 a and240 b and the excitation voltage, Ve, could be out of phase, additionalphase shifter 253 is used to equalize the phases. Output from the mixers254 a and 254 b are filtered by low pass filters 255 a and 255 b,respectively, to get an x-error voltage and a y-error voltage, themagnitudes of which represent the misalignment of the primary coil 130and the implanted device 100. The magnitude information is sent to theexternal charger 210's position indication circuitry 279, whereincalculations are performed to determine the degree and direction ofmisalignment 259 of the implanted device 100. Indication signals for theuser are then generated and sent to any of various display or indicationapparatuses, as are described below. Insert piece 241 and cylinderholders 242 and 243 are not shown in FIG. 6B for simplicity.

FIG. 6C shows a top down view of one of many possible mechanical designsfor the improved external charger apparatus pictured in FIGS. 6A and 6B.To detect small distortions within a strong magnetic field, fieldsensing coils 240 a and 240 b have to be precisely balanced. It may notbe mechanically practical to balance the halves of the sensing coils 240a and 240 b by physically moving or tilting them, although that ispossible. It may instead be easier to “tilt” the magnetic field of thesensing coils 240 a and 240 b. An insert, 241, typically a small pieceof material with high magnetic permeability such as ferrite or powderediron, can be used for this purpose. Placing the ferrite (or othersuitable material) insert 241 exactly into the geometrical center of thecoil system 240 does not affect field symmetry. However, moving theinsert piece 241 along the sensing coil 240 a or 240 b's axis changesfield “tilt” for that particular sensing coil, but has no effect on theother coil. So, tilting of the magnetic fields for both the x-axis coil240 b and y-axis coil 240 a can be achieved by moving insert piece 241directionally along either axis. This can be achieved by using arotatable insert piece holder 243. Insert piece holder 243 can be madewith two concentric cylinders. The smaller cylinder 242 could be rotatedinside the larger holder cylinder 243. The larger holder cylinder 243can then be rotated inside the overall coil system. Insert piece 241 ispreferably inserted into the small cylinder 242 asymmetrically, shiftedto the edge of the cylinder. The small cylinder 242 is then insertedasymmetrically into the larger holder cylinder 243. By rotating thesmaller cylinder 242, the displacement between the insert 241 and thecoil system's main axis can be changed from zero to a maximum value. Byrotating the larger holder cylinder 243, the direction of thedisplacement will be changed, and the sensing coils 240 a and 240 b maybe precisely balanced so that they are able to detect small distortionsin the reflected magnetic field 295.

FIG. 7 shows another embodiment of an improved external charger 210 withthe ability to determine the relative position of an implanted device,and thus maximize coupling by indicating to the user how to improvecharger/device alignment. Previous embodiments used x-y based sensors.However, position can also be “triangulated” using an embodiment withthree sensing coils positioned triangularly with respect to the centralaxis 275 of the primary coil. In such an embodiment, three field sensingcoils 230 are arranged within the primary coil 130 in a plane parallelto the primary coil 130. The field sensing coils 230 are designed todetect the amount of the reflected magnetic field 295 passing throughthem. The cross sectional areas of the sensing coils 230 should bemaximized to increase sensitivity to the reflected magnetic field 295.The coils 230 may be constructed either as air-core coils orferrite-core coils. By comparing the relative reflected magnetic fieldstrengths induced in the sensing coils 230, the position of the implantcan be “triangulated,” i.e., determined by the external charger 210'sposition indication circuitry 279 (FIGS. 5C-5F). Such triangulationtechniques may also be applied to the coils 240 of FIGS. 6A-6C.

FIG. 8 shows a block diagram of an improved alignment detection systemcomprising an improved external charger 210 for generating a magneticfield, including the field sensing coils 235 (which could consist ofeither field sensing coils 230, field sensing coils 240, or othersimilar coils), alignment sensing circuitry 281 for measuringreflections of the magnetic field, and the position indication circuitry279. The implantable device's circuitry 228 is similar to that describedin reference to FIG. 2 above, and is shown in a block for simplicity.Alignment sensing circuitry 281 comprises the circuitry for reading thefield sensing coils 235 and may be affixed to the PCB of the externalcharger 210, as mentioned above. Alignment sensing circuitry 281 sendsthe field sensing coil information to the position indication circuitry279, which discerns the alignment between the implanted device 100 andthe external charger 210. Position indication circuitry 279 thenindicates to the user a direction in which the external charger 210should be moved to improve the alignment of the external charger 210relative to the implantable medical device 100. Such indication mayoccur in a variety of ways, including, but not limited to: activatingvisual indicators, such as LED lights 295 which can be configured tolight up on the surface of the external charger 210 (See FIG. 10);activating audible indicators, such as beeps or verbal commands to theuser; or activating tactile indicators, such as vibrating certain sidesof the external charger 210 to indicate that the external charger 210needs to be moved in that direction.

Because external charger 210 is often placed against a patient's back orbuttocks, it can be difficult for the patient to receive informationfrom the external charger 210 indicating how to improve the charger'salignment. To provide better positioning information to the patient, theexternal charger 210 may optionally transmit, via communications link250, misalignment information to another external device for controllingthe therapeutic settings of the implantable medical device, e.g., remotecontrol 218. The external device may then indicate how the externalcharger 210 should be moved to improve the alignment of the externalcharger 210 relative to the implantable medical device 100. This type ofcommunication is disclosed in commonly-owned U.S. patent applicationSer. No. 12/476,523, filed Jun. 2, 2009.

FIG. 9 is a flowchart detailing one embodiment of a technique forassuring the proper alignment of an external charger 210 to an IPG 100.First, the user places external charger 210 against the surface of hisbody 278 in the known vicinity of IPG 100 (310). At this time, thepatient will activate the external charger 210 and begin charging IPG100 (320). The default setting for external charger 210 is maximum poweroutput. As long as external charger doesn't receive an indication thatIPG 100 is fully charged (330), it will continue to charge IPG 100. Asexternal charger 210 is charging IPG 100, alignment sensing circuitry281 in the external charger 210 senses the charger's alignment with theIPG 100 based at least in part on electrical measurements taken from theplurality of sensing coils 235 in the external charger 210, and positionindication circuitry 279 calculates the IPG 100's location (340). Thiscalculation occurs in real time (340) so that, any time alignmentbecomes poor, corrective action can be indicated to the user and takenin subsequent steps. If IPG 100 and the external charger 210 areproperly aligned (350), external charger 210 continues to charge the IPG100's internal power source 180 until receiving indication that IPG 100is fully charged (330). If the external charger 210 determines that IPG100 and the external charger 210 are not properly aligned (350), theexternal charger 210 will indicate to the user (via one of the variousmethods discussed above) which direction to move the external charger210 to improve alignment (360) while still continuing to charge IPG 100.Once the external charger 210 determines that the IPG 100's internalpower source 180 is fully charged (330), it will indicate via an audiblebeep or other visual indication to the user that the charging processhas completed (370).

FIG. 10 shows one embodiment of an improved external charger 210 forcharging an implantable device. The external charger 210 is shownsitting in a base unit 296. In this embodiment, four arrow-shaped LEDlights 297 are arranged on the surface of the external charger 210, withone arrow-shaped LED light pointing towards each edge of externalcharger 210. As position indication circuitry 279 determines in whichdirection the external charger 210 should be moved to provide betteralignment with implantable device 100, it can send an appropriatecontrol signal to illuminate one or more of the LED lights 297 toindicate that direction to the user. When position determinationcircuitry 279 has detected that there is a satisfactory degree ofalignment between the external charger 210's primary coil 130 and theimplantable device, position indication circuitry 279 will send acontrol signal to turn off each LED light 297 until it again senses amisalignment condition during charging.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An external charger for issuing a magnetic fieldto charge an implantable medical device, comprising: a plurality ofsensors; a primary coil for issuing the magnetic field to charge theimplantable medical device wherein the plurality of sensors are locatedwithin a center of the primary coil; and an alignment sensing circuitconfigured to determine an alignment of the external charger relative tothe implantable medical device, wherein the determination is based onelectrical measurements taken from the plurality of sensors in responseto an interaction between the magnetic field and the implantable medicaldevice.
 2. The external charger of claim 1, further comprising: aposition indication circuit coupled to the alignment sensing circuitconfigured to indicate to a user a misalignment of the external chargerrelative to the implantable medical device.
 3. The external charger ofclaim 2, wherein the position indication circuit is further configuredto indicate to the user how to improve the alignment of the externalcharger relative to the implantable medical device.
 4. The externalcharger of claim 2, wherein the position indication circuit isconfigured to activate visual indicators on the external charger.
 5. Theexternal charger of claim 4, wherein the visual indicators areconfigured to indicate a direction in which the external charger shouldbe moved to improve the alignment of the external charger relative tothe implantable medical device.
 6. The external charger of claim 2,wherein the position indication circuit is configured to transmitmisalignment information to another external device.
 7. The externalcharger of claim 1, wherein the electrical measurements comprise avoltage produced by the plurality of sensors.
 8. The external charger ofclaim 1, wherein the primary coil is wound around a central axis,wherein the plurality of sensors comprise a plurality of sensing coils,and wherein the plurality of sensing coils are wound around axes thatare parallel to the central axis.
 9. The external charger of claim 1,wherein the primary coil is wound around a central axis, wherein theplurality of sensors comprise a plurality of sensing coils, and whereinthe plurality of sensing coils are wound around axes that areperpendicular to the central axis.
 10. The external charger of claim 1,wherein the plurality of sensors comprises at least one pair of sensors,wherein the sensors in each pair straddle a central axis of the primarycoil.
 11. The external charger of claim 10, wherein the plurality ofsensors comprise a plurality of sensing coils, wherein the sensing coilsin each pair are connected to each other.
 12. The external charger ofclaim 1, wherein the plurality of sensors comprise three sensing coils,wherein the three sensing coils are positioned triangularly with respectto a central axis of the primary coil.
 13. The external charger of claim1, wherein the primary coil is wound around a first axis, wherein theplurality of sensors comprise a plurality of sensing coils, and whereinthe plurality of sensing coils are wound around second axes that lay ina plane of the primary coil.
 14. The external charger of claim 1,wherein the plurality of sensors comprise a sensing coil divided intotwo separate parts connected by a wire, and wherein the determination isbased on electrical measurements taken from each of the parts.
 15. Anexternal charger for issuing a magnetic field to charge an implantablemedical device, comprising: a primary coil for issuing a magnetic fieldto charge the implantable medical device wherein the primary coil iswound around a central axis; a plurality of sensors oriented to sensemagnetic fields along second axes, wherein each second axis isperpendicular to the central axis; and an alignment sensing circuitconfigured to determine an alignment of the external charger relative tothe implantable medical device, wherein the determination is based onelectrical measurements taken from the plurality of sensors in responseto an interaction between the magnetic field and the implantable medicaldevice.
 16. The external charger of claim 15, further comprising: aposition indication circuit coupled to the alignment sensing circuitconfigured to indicate to a user a misalignment of the external chargerrelative to the implantable medical device.
 17. The external charger ofclaim 16, wherein the position indication circuit is further configuredto indicate to the user how to improve the alignment of the externalcharger relative to the implantable medical device.
 18. The externalcharger of claim 16, wherein the position indication circuit isconfigured to activate visual indicators on the external charger. 19.The external charger of claim 18, wherein the visual indicators areconfigured to indicate a direction in which the external charger shouldbe moved to improve the alignment of the external charger relative tothe implantable medical device.
 20. The external charger of claim 16,wherein the position indication circuit is configured to transmitmisalignment information to another external device.
 21. The externalcharger of claim 15, wherein the electrical measurements comprise avoltage produced by the plurality of sensors.
 22. The external chargerof claim 15, wherein the plurality of sensors comprise a plurality ofsensing coils each wound around one of the second axes.
 23. The externalcharger of claim 15, wherein the plurality of sensors comprises at leastone pair of sensors, wherein the sensors in each pair straddle thecentral axis of the primary coil.
 24. The external charger of claim 23,wherein the sensors in each pair are connected to each other.
 25. Theexternal charger of claim 15, wherein the plurality of sensors comprisethree sensing coils, wherein the three sensing coils are positionedtriangularly with respect to the central axis of the primary coil. 26.The external charger of claim 15, wherein the second axes lay in a planeof the primary coil.
 27. The external charger of claim 15, wherein theplurality of sensors comprise a sensing coil divided into two separateparts connected by a wire, and wherein the determination is based onelectrical measurements taken from each of the parts.